First, there are many grading companies out there. In order for the graded coin to be accepted as being the grade it is claimed to be the person buying the coin must respect the grading company’s work. Most coin collectors will readily accept PCGS, NGC, ANACS, and IGC. PCGS is highly recognized, and NGC has a relationship with the American Numismatics Association.
ANACS was the first grading service in response to the need of collectors to authenticate coins as genuine, and is still highly respected as the authority on ancient coins. ICG is usually the grading services used for many Canadian coins.